Detectors for determining the properties of a road surface are well known in the art, including stationary detectors arranged along the roadside and above the surface, detectors arranged beneath the road surface as well as detectors arranged on the vehicles.
A simple principle for contact-less measurement is disclosed in U.S. Pat. No. 4,274,091 by Decker, in which a series of light pulses is directed towards the road surface by a light emitter with a chopper wheel, and the intensity of the reflected light is measured with one receiver. The amplitude of the signal measured by the receiver indicates the presence or absence of ice on the road surface.
Another principle is disclosed in U.S. Pat. No. 4,690,553 by Fukamizu et al., in which an infrared light emitter directs a ray towards the road surface, and two receivers are arranged to measure the mirror reflected light and the scattered or diffuse reflected light, respectively. The ratio of the output from the two receivers provides information of the road surface properties. Utilisation of the reflection of infrared light to measure the road surface properties is also known from e.g. DE 2,712,199, EP 0,005,696 and DE 3,023,444.
Other principles involves the use of acoustic waves as disclosed in Japanese patent application JP 03-110408 by Hiroshi and Masami, The use of microwaves is disclosed e.g. in U.S. Pat. No. 5,652,522 by Kates and Butler and in U.S. Pat. No. 3,836,846 by Overall and deKoning, the use of microwaves in combination with a laser light beam as disclosed in Japanese patent application JP 06-307838 by Takaharu, the use of multiple wavelength regions of infrared light is disclosed in U.S. Pat. No. 5,218,206 and in U.S. Pat. No. 5,962,853.
All the disclosed measurement principles and other similar principles may be utilised for the device and the system of the present invention. The different arrangements of the sensors have each their drawback. Sensors arranged along the roadside and above the road surface have to be about 4.5 meters above to road surface to allow all types of vehicles to pass, and the air between the sensor and the road surface will be contaminated with particles raised from the road surface by the passing vehicles, in particular when the road surface is wet or covered with sled or snow, and the reliability of the sensor is therefore low under the weather conditions at which the warnings of slippery surface conditions are of importance. Sensors embedded in the road surface and measuring the properties thereof from below through a transparent lid, suffer from contamination of the upper surface of the lid and development of scratches, which ruin the operation of the sensor. Sensors mounted on the vehicle measure from above close to the road surface, but provide only information about the road surface properties at the position of the vehicle, which often is too late for the driver of the vehicle to take measures, in particular under changing properties of the road surfaces and during weather conditions where only local areas of the road surfaces have slippery conditions.